SYNTHESIS OF AMORPHOUS CALCIUM-CARBONATE AND ITS CRYSTALLIZATION

The synthetic conditions of amorphous calcium carbonate such as initial concentration of reactant, temperature and pH in the reaction of CaCl2-Na2CO3-NaOH system, and its crystallization in water and NH4 Cl solution were investigated. Characteristics of the amorphous phase were determined by means of X-ray diffraction, thermal analysis (TG-DTA), scanning electron microscopic observation and measurement of solubility. The amorphous calcium carbonate (ACC) and amorphous basic calcium carbonate (ABCC) were precipitated by adding 0.1 mol.dm-3 CaCl2 solution into the mixed solution of 0-3.0 mol.dm-3 NaOH and 0.1 mol.dm-3 Na2CO3, and then the precipitated colloidal phases were filtered and dried rapidly through dry air under reduced pressure. The stable phases of ACC and ABCC were formed in the region of pH 11-14 at 0-degrees-C but the formation regions of both amorphous phases were remarkably affected by pH in the mother liquor. Namely, ACC was formed at pH 11.0-13.5 and ABCC at pH 13.5-14.0. Their compositions were formulated as CaCO3.1.5H2O for the former and Ca3 (OH)6X (CO3)3-3x.3yH2O (0 < X less-than-or-equal-to 0.2, 1.5 > Y greater-than-or-equal-to 1.2) for the latter. These results show that increase of pH contributed to the partial replacement Of CO32- of ACC by OH-, which resulted in the formation of ABCC with different compositions. ACC was dissolved rapidly in water and its highest solubility was 1.8 x 10(-4) mol Ca2+/100 CM3 H2O, i.e. ten times larger than that of a commercial calcium carbonate (calcite-type). The dissolution of ABCC was affected by the amount of OH- contained in the random structure and the highest solubility of ABCC with composition Ca3(OH)0.72(CO3)2.64.4.14H2O was 4.0 x 10(-4) mol Ca2+/100 cm3 H2O after dipping in water for 10 min. The control of crystal shape and modification of calcium carbonate were investigated by dipping spherical secondary-particles of ACC in water and 1 mol.dm-3 NH4Cl aqueous solution, respectively. ACC was crystallized into rhombohedral-like calcite with about 4 mum in size at 20-degrees-C, needle-like aragonite with length 1-3 mum and diameter 0.2 mum at 80-degrees-C when dipped in water and spherical vaterite with diameter 0.5-1.5 mum in 1 mol.dm-3 NH4Cl aqueous solution.